arXiv subjects

Alfredo Herrera-Aguilar

Publications and source records attributed to Alfredo Herrera-Aguilar.

At least 37 records · Page 2Linked to original sources

arXiv2021

Anisotropic Schrödinger black holes with hyperscaling-violation

We investigate novel exact solutions to an Einstein-Maxwell theory non-minimally coupled to a self-interacting dilaton-like scalar. Extending the results of arXiv: 2012.13412 and 2110.04445, we report three families of exact configurations over a non-relativistic Schrödinger background with both, arbitrary dynamical critical exponent $z$ and hyperscaling violating parameter $θ$. Concretely, we provide field configurations with hyperscaling violation which are asymptotically Schrödinger spaces. Our solutions correspond to three kinds: a zero-temperature background, a naked singularity and, more interestingly, a family of black holes. To the later, we construct the corresponding Carter-Penrose diagram with a view to understand their causal structure given the non-standard background. We show that a non-trivial hyperscaling violation parameter $θ$ is necessary in order to support a real non-constant dilaton field in the configuration. We explore how the relation between the hyperscaling violation parameter and the critical dynamical exponent determine, in combination with the spacetime dimension, the kinematic aspects of the fields. We provide a thorough study of the thermodynamics including the quasi-local computation of charges, the verification of the first law and arguments concerning the stability. Lastly, we explore the effects in the thermodynamics from varying the rich parameter space. We pay special attention in comparing the qualitative behavior of the thermodynamics scalar-free solutions and the ones with a nontrivial dialton.

hep-th↗

arXiv2021

Rotating Spacetimes generalizing Lifshitz Black Holes

We present a spinning black hole solution in $d$ dimensions with a maximal number of rotation parameters in the context of the Einstein-Maxwell-Dilaton theory. An interesting feature of such a solution is that it accommodates Lifshitz black holes when the rotation parameters are set to zero. We verify the rotating nature of the black hole solution by performing the quasi-local analysis of conserved charges and defining the corresponding angular momenta. In addition, we perform the thermodynamical analysis of the black hole configuration, show that the first law of thermodynamics is completely consistent, and obtain a Smarr-like formula. We further study the thermodynamic stability of the constructed solution from a local viewpoint, by computing the associated specific heats, and from a global perspective, by using the so-called new thermodynamic geometry. We finally make some comments related to a pathology found in the causal structure of the obtained rotating black hole spacetime and compute some of its curvature invariants.

hep-th↗

arXiv2021

Hyperscaling violating Schrodinger black holes in Einstein-Maxwell-scalar theory

We present a novel family of asymptotically Schrodinger hyperscaling violating black holes with a generic dynamical critical exponent and an arbitrary number of spacetime dimensions. This black hole family represents a solution within the Einstein-Maxwell-scalar setup with a self-interaction scalar potential where the Maxwell field is coupled to the scalar field. Through an analysis of the curvature invariants it is observed that this configuration is asymptotically regular for different ranges of the hyperscaling violating exponent. Furthermore, the above mentioned solution constitutes a gravitational candidate for describing field theories with hyperscaling violating Schrodinger symmetry at finite temperature within the framework of the Gravity/Condensed Matter Theory correspondence.

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arXiv2021

Toward the Gravitational Redshift Detection in NGC 4258 and the Estimation of its Black Hole Mass-to-Distance Ratio

We construct from first principles a general relativistic approach to study Schwarzschild black hole (BH) rotation curves and estimate the mass-to-distance ratio of the active galactic nucleus of NGC 4258 in terms of astrophysical observable quantities. The presented method allows one to clearly distinguish and quantify the general and special relativistic contributions to the total redshift expression. The total relativistic redshift/blueshift comprises three components: the gravitational redshift due to the spacetime curvature generated by the mass of the BH in its vicinity, the kinematic shift, originated by the photons' local Doppler effect, and the redshift due to a special relativistic boost that describes the motion of a galaxy from a distant observer. We apply our method to the largest data set of highly redshifted water megamaser measurements on the accretion disk of the NGC 4258 active galaxy and use this general relativistic method to estimate its BH mass-to-distance ratio: M/D = (0.5326 +/- 0.00022) x 10^7 solar masses/Mpc.

gr-qc↗

arXiv2021

Scalarization-like mechanism through spacetime anisotropic scaling symmetry

We present a new family of exact black hole configurations, which is a solution to a generalized Einstein-Maxwell-Dilaton setup in arbitrary dimension. These solutions are asymptotically Lifshitz for any dynamical critical exponent $z\geq 1$. It turns out that the existence of a nontrivial scalar field is a direct consequence of breaking the spacetime isotropic scaling symmetry. This black hole family accepts various interesting limits that link it to well-known solutions in both the isotropic and anisotropic cases. We study the thermodynamics of these field configurations showing that the first law is satisfied and providing the corresponding Smarr formula, both of these relations account for an electric contribution. Furthermore, we show that for a certain parameter region, the anisotropic field configuration with a nonzero scalar field is thermodynamically preferred. This observation, together with a direct verification of the so-called scalarization conditions, suggest that the emergence of the dilaton field is due to a mechanism similar to spontaneous scalarization.

hep-th↗

arXiv2021

General relativistic formulas for mass and spin of a Kerr black hole in terms of redshifts and orbital parameters

We derive closed formulas for the mass and spin parameters of a Kerr black hole in terms of a minimal quantity of observational data: the red-/blue-shifts of photons emitted by massive particles (stars) moving on geodesics around the black hole and their respective orbital radius. It turns out that given a set of two (three) stars revolving around the black hole, these formulas involve just eight (twelve) observational data. For the case of a single star orbiting the black hole we need a minimal set of four observational measurements to analytically determine both parameters.

gr-qc↗

arXiv2020

Anisotropic Lifshitz holography in Einstein-Proca theory with stable negative mass spectrum

In this article we focus on constructing a new family of spatially anisotropic Lifshitz spacetimes with arbitrary dynamical exponent z and constant negative curvature in d+1 dimensions within the framework of the Einstein-Proca theory. The constructed metric tensor depends on both the spacetime dimensionality and the critical exponent, while the curvature scalar depends just on the number of dimensions. We also obtained a novel spectrum with negative squared mass that respects the corresponding Breitenlohner-Freedman bound. Hence these new solutions are stable and can be useful within the gravity/condensed matter theory holographic duality, since the spectrum with negative squared mass is complementary to the positive ones already known in the literature.

hep-th↗

arXiv2018

Mass and spin of a Kerr black hole in modified gravity and a test of the Kerr black hole hypothesis

In this paper we compute the Arnowitt-Deser-Misner (ADM) mass, the angular momentum and the charge of the Kerr black hole solution in the scalar-tensor-vector gravity theory [known as the Kerr-MOG (modified-gravity) black hole configuration]; we study in detail as well several properties of this solution such as the stationary limit surface, the event horizon, and the ergosphere, and conclude that the new deformation parameter $α$ affects the geometry of the Kerr-MOG black hole significantly in addition to the ADM mass and spin parameters. Moreover, the ADM mass and black hole event horizon definitions allow us to set a novel upper bound on the deformation parameter and to reveal the correct upper bound on the black hole spin. We further find the geodesics of motion of stars and photons around the Kerr-MOG black hole. By using them we reveal the expressions for the mass and the rotation parameter of the Kerr-MOG black hole in terms of the red- and blueshifts of photons emitted by geodesic particles, i.e., by stars. These calculations supply a new and simple method to further test the general theory of relativity in its strong field limit: If the measured red- and blueshifts of photons exceed the bounds imposed by the general theory of relativity, then the black hole is not of Kerr type. It could also happen that the measurements are allowed by the Kerr-MOG metric, implying that the correct description of the dynamics of stars around a given black hole should be performed using MOG or another modified theory of gravity that correctly predicts the observations. In particular, this method can be applied to test the nature of the putative black hole hosted at the center of the Milky Way in the near future.

gr-qc↗

arXiv2017

General bounds in Hybrid Natural Inflation

Recently we have studied in great detail a model of Hybrid Natural Inflation (HNI) by constructing two simple effective field theories. These two versions of the model allow inflationary energy scales as small as the electroweak scale in one of them or as large as the Grand Unification scale in the other, therefore covering the whole range of possible energy scales. In any case the inflationary sector of the model is of the form $V(ϕ)=V_0 \left(1+a \cos(ϕ/f)\right)$ where $0\leq a<1$ and the end of inflation is triggered by an independent waterfall field. One interesting characteristic of this model is that the slow-roll parameter $ε(ϕ)$ is a non-monotonic function of $ϕ$ presenting a {\it maximum} close to the inflection point of the potential. Because the scalar spectrum $\mathcal{P}_s(k)$ of density fluctuations when written in terms of the potential is inversely proportional to $ε(ϕ)$ we find that $\mathcal{P}_s(k)$ presents a {\it minimum} at $ϕ_{min}$. The origin of the HNI potential can be traced to a symmetry breaking phenomenon occurring at some energy scale $f$ which gives rise to a (massless) Goldstone boson. Non-perturbative physics at some temperature $T<f$ might occur which provides a potential (and a small mass) to the originally massless boson to become the inflaton (a pseudo-Nambu-Goldstone boson). Thus the inflaton energy scale $Δ$ is bounded by the symmetry breaking scale, $Δ\equiv V_H^{1/4} <f.$ To have such a well defined origin and hierarchy of scales in inflationary models is not common. We use this property of HNI to determine bounds for the inflationary energy scale $Δ$ and for the tensor-to-scalar ratio $r$.

astro-ph.CO↗

arXiv2017

A de Sitter tachyonic braneworld revisited

Within the framework of braneworlds, several interesting physical effects can be described in a wide range of energy scales, starting from high-energy physics to cosmology and low-energy physics. An usual way to generate a thick braneworld model relies in coupling a bulk scalar field to higher dimensional warped gravity. Quite recently, a novel braneworld was generated with the aid of a tachyonic bulk scalar field, having several remarkable properties. It comprises a regular and stable solution that contains a relevant 3--brane with de Sitter induced metric, arising as an exact solution to the 5D field equations, describing the inflationary eras of our Universe. Besides, it is {\it asymptotically flat}, despite of the presence of a negative 5D cosmological constant, which is an interesting feature that contrasts with most of the known, asymptotically either dS or AdS models. Moreover, it encompasses a graviton spectrum with a single massless bound state, accounting for 4D gravity localized on the brane, separated from the continuum of Kaluza-Klein massive graviton modes by a mass gap that makes the 5D corrections to Newton's law to decay exponentially. Finally, gauge, scalar and fermion fields are also shown to be localized on this braneworld. In this work, we show that this tachyonic braneworld allows for a nontrivial solution with a vanishing 5D cosmological constant that preserves all the above mentioned remarkable properties with a less amount of parameters, constituting an important contribution to the construction of a realistic cosmological braneworld model.

hep-th↗

arXiv2017

Symplectic analysis of three dimensional Abelian topological gravity

A detailed Faddeev-Jackiw quantization of an Abelian topological gravity is performed; we show that this formalism is equivalent and more economical than Dirac's method. In particular, we identify the complete set of constraints of the theory, from which the number of physical degrees of freedom is explicitly computed. We prove that the generalized Faddeev-Jackiw brackets and the Dirac ones coincide to each other. Moreover, we perform the Faddeev-Jackiw analysis of the theory at the chiral point, and the full set of constraints and the generalized Faddeev-Jackiw brackets are constructed. Finally we compare our results with those found in the literature and we discuss some remarks and prospects.

hep-th↗

arXiv2016

Canonical single field slow-roll inflation with a non-monotonic tensor-to-scalar ratio

We take a pragmatic, model independent approach to single field slow-roll canonical inflation by imposing conditions, not on the potential, but on the slow-roll parameter $ε(ϕ)$ and its derivatives $ε^{\prime }(ϕ)$ and $ε^{\prime\prime }(ϕ)$, thereby extracting general conditions on the tensor-to-scalar ratio $r$ and the running $n_{sk}$ at $ϕ_{H}$ where the perturbations are produced, some $50$ $-$ $60$ $e$-folds before the end of inflation. We find quite generally that for models where $ε(ϕ)$ develops a maximum, a relatively large $r$ is most likely accompanied by a positive running while a negligible tensor-to-scalar ratio implies negative running. The definitive answer, however, is given in terms of the slow-roll parameter $ξ_2(ϕ)$. To accommodate a large tensor-to-scalar ratio that meets the limiting values allowed by the Planck data, we study a non-monotonic $ε(ϕ)$ decreasing during most part of inflation. Since at $ϕ_{H}$ the slow-roll parameter $ε(ϕ)$ is increasing, we thus require that $ε(ϕ)$ develops a maximum for $ϕ> ϕ_{H}$ after which $ε(ϕ)$ decrease to small values where most $e$-folds are produced. The end of inflation might occur trough a hybrid mechanism and a small field excursion $Δϕ_e\equiv |ϕ_H-ϕ_e |$ is obtained with a sufficiently thin profile for $ε(ϕ)$ which, however, should not conflict with the second slow-roll parameter $η(ϕ)$. As a consequence of this analysis we find bounds for $Δϕ_e$, $r_H$ and for the scalar spectral index $n_{sH}$. Finally we provide examples where these considerations are explicitly realised.

astro-ph.CO↗

arXiv2016

Stability of a tachyon braneworld

Within the braneworld paradigm the tachyonic scalar field has been used to generate models that attempt to solve some of the open problems that physics faces nowadays, both in cosmology and high energy physics as well. When these field configurations are produced by the interplay of higher dimensional warped gravity with some matter content, braneworld models must prove to be {\it stable} under the whole set of small fluctuations of the gravitational and matter fields background, among other consistency tests. Here we present a complete proof of the stability under scalar perturbations of tachyonic thick braneworlds with an embedded maximally symmetric 4D space-time, revealing its physical consistency. This family of models contains a recently reported tachyonic de Sitter thick braneworld which possesses a series of appealing properties. These features encompass complete regularity, asymptotic flatness (instead of being asymptotically dS or AdS) even when it contains a negative bulk cosmological constant, a relevant 3-brane with dS metric which naturally arises from the full set of field equations of the 5D background (it is not imposed), qualitatively describing the inflationary epochs of our Universe, and a graviton spectrum with a single zero mode bound state that accounts for the 4D graviton localised on the brane and is separated from the continuum of Kaluza-Klein massive graviton excitations by a mass gap. Gauge vector fields with a single massless bound state in its mass spectrum are also localised on this braneworld model a fact that allows us to recover the Coulomb's law of our 4D world. All these properties of the above referred tachyonic braneworld together with the positive stability analysis provided in this work, constitute a firm step towards the construction of realistic cosmological models within the braneworld paradigm.

hep-th↗

arXiv2015

Kerr black hole parameters in terms of red/blue shifts of photons emitted by geodesic particles

We are motivated by the recently reported dynamical evidence of stars with short orbital periods moving around the center of the Milky Way and the corresponding hypothesis about the existence of a supermassive black hole hosted at its center. In this paper we show how the mass and rotation parameters of a Kerr black hole (assuming that the putative supermassive black hole is of this type), as well as the distance that separates the black hole from the Earth, can be estimated in a relativistic way in terms of i) the red and blue shifts of photons that are emitted by geodesic massive particles (stars and galactic gas) and travel along null geodesics towards a distant observer, and ii) the radius of these star/gas orbits. As a concrete example and as a first step towards a full relativistic analysis of the above mentioned star orbits around the center of our galaxy, we consider stable equatorial circular orbits of stars and express their corresponding red/blue shifts in terms of the metric parameters (mass and angular momentum per unit mass) and the orbital radii of both the emitter star (and/or galactic gas) and the distant observer. In principle, these expressions allow one to statistically estimate the mass and rotation parameters of the Kerr black hole, and the radius of our orbit, through a Bayesian fitting, i.e., with the aid of observational data: the red/blue shifts measured at certain points of stars' orbits and their radii, with their respective errors, a task that we hope to perform in the near future. We also point to several astrophysical phenomena, like accretion discs of rotating black holes, binary systems and active galactic nuclei, among others, to which this formalism can be applied.

gr-qc↗

arXiv2015

Constraining Hybrid Natural Inflation with recent CMB data

We study the Hybrid Natural Inflation (HNI) model and some of its realisations in the light of recent CMB observations, mainly Planck temperature and WMAP-9 polarization, and compare with the recent release of BICEP2 dataset. The inflationary sector of HNI is essentially given by the potential $V(ϕ) = V_0(1+a\cos (\fracϕ{f} ) )$, where $a$ is a positive constant smaller or equal to one and $f$ is the scale of (pseudo Nambu-Goldstone) symmetry breaking. We show that to describe the HNI model realisations we only need two observables; the spectral index $n_s$, the tensor-to-scalar ratio, and a free parameter in the amplitude of the cosine function $a$. We find that in order to make the HNI model compatible with the BICEP2 observations, we require a large positive running of the spectra. We find that this could over-produce primordial black holes in the most consistent case of the model. This situation could be aleviated if, as recently argued, the BICEP2 data do not correspond to primordial gravitational waves.

astro-ph.CO↗

arXiv2014

Coulomb's law corrections and fermion field localization in a tachyonic de Sitter thick braneworld

In this work, following recent studies which show that it is possible to localize gravity as well as scalar and gauge vector fields in a tachyonic de Sitter thick braneworld, we investigate the localization of fermion fields in this model. In order to achieve this aim we consider the Yukawa interaction term between the fermions and the tachyonic condensate scalar field MF(T)barPsiPsi in the action and analyze four different cases corresponding to distinct tachyonic functions F(T(w)). The only condition that this function must satisfy in order to yield 4D chiral fermions upon dimensional reduction is to be odd in the extra dimension w. These functions lead to a different structure of the respective fermionic mass spectrum. In particular, localization of the massless left-chiral fermion zero mode is possible for three of these cases. We further analyze the phenomenology of the Yukawa interaction among fermion fields and gauge bosons localized on the brane and obtain the crucial and necessary information to compute the corrections to Coulomb's law coming from massive KK vector modes in the non-relativistic limit. These corrections are exponentially suppressed due to the presence of the mass gap in the mass spectrum of the bulk gauge vector field. From our results we conclude that corrections to Coulomb's law in the thin brane limit have the same form (up to a numerical factor) as far as the left-chiral massless fermion field is localized on the brane. Finally we compute the corrections to the Coulomb's law for a more general case, on a thick brane scenario, for which we can do estimates consistent with brane phenomenology, i.e. we found that the predicted corrections to the Coulomb law in our model, which are well bounded by the observed experimental photon mass, is far beyond its upper bound, positively testing the viability of our tachyonic braneworld.

hep-th↗

arXiv2014

Discrete graviton spectrum from super-exponential cup potentials and their application to braneworld physics

Super-exponential warp factors have been proposed in the context of two-brane models to solve the gauge hierarchy problem of scales by using a compactification scale of the same order of the fundamental Planck scale, completely eliminating the hierarchy between the electroweak and the Planck scales. However, most of the so far studied families of braneworlds with super-exponential warp factors do not localize 4D gravity when one of the branes is sent to infinity. Recently a braneworld model generated by a canonical scalar field $ϕ$ minimally coupled to 5D gravity with a bulk cosmological constant was shown to localize 4D gravity and to be stable under linear tensorial and scalar perturbations. Here we present an explicit new solution for the latter braneworld configuration and study the dynamics of its tensorial perturbations: we find that they obey a Schrödinger-like equation with a well potential which possesses exponentially increasing walls (we call it the {\it cup potential}), yielding a {\it novel discrete spectrum} for the massive Kaluza-Klein (KK) tensorial excitations, despite the non-compact nature of the fifth dimension, and in contrast to the braneworld models proposed so far, where the mass spectrum of the KK modes is continuous, or at most mixed. Finally, the corrections to Newton's law coming from these massive KK modes are computed. Their novelty arises from the fact that these {\it KK excitations possess quantized masses and are bound to the brane}, however, they are all of the Planck mass order, leading to unreachable energy scales from the phenomenological viewpoint.

hep-th↗

arXiv2014

Testing Hybrid Natural Inflation with BICEP2

We analyse Hybrid Natural Inflation in view of the recent results for the tensor index reported by BICEP2. We find that it predicts a large running of the scalar spectrum which is potentially detectable by large scale structure and $21\, \mathrm{cm} $ observations. The running of the running is also relatively large becoming close to $10^{-2}$. Along the way, we find general consistency relations at which observables are subject if the slow-roll approximation is imposed. Failure to satisfy these equations by the values obtained for the observables in surveys would be a failure of the slow-roll approximation itself.

astro-ph.CO↗